The three reactions of the IS cycle, which is disclosed in U.S. Pat. No. 4,089,940, are as follows:H2SO4→SO2+H2O+1/202 SO2+I2+2H2O→2HI+H2SO4 2HI→I2+H2 
The decomposition of sulfuric acid into sulfur dioxide and oxygen is performed at about 700° C. or above. Sulfur dioxide reacts with iodine and water (Bunsen reaction) to produce hydrogen iodide and sulfuric acid. Sulfuric acid is recycled to the sulfuric acid decomposition reaction. Hydrogen iodide is separated and decomposed in a hydrogen iodide reactor to generate hydrogen and iodine. Iodine is recycled to the Bunsen reactor. As a whole, this process is a closed cycle whose net reactant is water and whose net products are hydrogen and oxygen.
As described, in the process of absorption, separation and stripping of SO2 at high temperature in the thermochemical hydrogen production by the IS cycle, SO2 produced from the decomposition of H2SO4 is separated and recycled as a reactant to the Bunsen reaction (120° C.). Unlike the conventional desulfurization from the exhaust gases, reversible absorption of SO2 is essential in this process because SO2 is recycled. The recovered SO2 gas should be pure not containing solvent vapor, and the stripping of SO2 at high temperature should occur continuously and stably for the better thermal efficiency.
In general, SO2 absorbents can be regenerated by the application of sufficient heat or chemical reagents. At present, steam stripping at about 90° C., calcination at about 900 to 1,000° C., reduction with reductants such as hydrogen, methane and carbon monoxide, reaction with lime and electrodialysis/oxidation are known.
Various absorbents have been used as SO2 absorbents. U.S. Pat. No. 3,904,735 describes use of triethanolamine. A variety of other amine-based compounds have been used for SO2 removal. Because of the relatively high vapor pressure of amine-based compounds at high temperature, the stripping temperature of SO2 should be increased around 110° C. to obtain pure SO2 not containing the amine-based compounds.
According to the Waterloo scrubber process disclosed in U.S. Pat. No. 4,067,703, any amine absorbent used for SO2 absorption cannot avoid the loss of solvent due to potential evaporation caused by high vapor pressure and large surface area of the absorption medium. U.S. Pat. No. 5,019,361 (Hakka) discloses the removal of SO2 using a diamine compound having a pKa value of about 4.5 to 6.7. The amine salt resulting from absorption at 25 to 70° C. may be regenerated to SO2 by steam in a stripper tower, as disclosed in U.S. Pat. No. 5,292,407 (Roy et al.). The degassing temperature is maintained at 100 to 110° C.
Certain problems may result from the use of the aforesaid absorbents. For example, the stripping of SO2 should be performed at 100 to 110° C., and because of the high vapor pressure of the absorbents at high temperature, the purity of the recycled SO2 tends to be low.